Synthesis, characterization and modification of LiFePO4 by doping with platinum and palladium for lithium-ion batteries

Lithium iron phosphate (LiFePO4) with features of excellent thermal stability, non-toxicity, low cost and abundance in nature is one of the most promising cathode materials to be used in lithium ion batteries. However, as it suffers from the low electrical conductivity and poor ionic diffusion, it operates only at low charge/discharge current rates. In this thesis, a dual approach of metal doping and carbon coating was employed to solve the aforementioned problem. This work is mainly on the study, for the first time, of the effect of platinum and palladium doping of LiFePO 4 on its physical-chemical properties. The effect of Pt and Pd doping on the LiFePO4 performance as Li-ion cathode will be also shown. Sol-gel and hydrothermal methods were used to synthesize the LiFePO4 and doped-LiFePO4 cathode materials. The prepared materials were characterized using different methods such as XRD (X-ray Diffraction), XPS (X-ray Photoelectron Spectroscopy), SEM (Scanning Electron Microscopy) and BET (Brunauer Emmett Teller). The electrochemical characterization techniques including charge/discharge test, CV (Cyclic Voltammetry), EIS (Electrochemical Impedance Spectroscopy) and cycling were also used. The effects of metals doping on chemical-physical properties, particles sizes, morphology, structure and purity of the electrodes were investigated and their correlation to the electrochemical properties of materials were studied.;In the second part, the effect of doping the LiFePO4/C electrodes with Pt and Pd on the structure, their chemical compositions and electrochemical properties were investigated. The electrodes were prepared using sol-gel method. The chemical composition analysis by XPS of the Pd doped electrodes showed that Pd was detected in the Pd-doped nano composite materials. Also, for Pt doped electrode, Pt was detected in Pt-doped based nano composite materials. Based on the structure and morphology of the non doped and doped electrodes, the results showed that palladium doping facilitated formation of Li 3PO4 impurity in the LiFe0.98Pd0.02PO 4/C and LiFe0.96Pd0.04PO4/C electrodes. Consequently the presence of the Pd in the doped sample reduced the lattice parameters, increased the size of particles and caused their agglomeration. The electrochemical performances of the electrodes showed that their specific capacity decreased when the palladium content increases. Then, the Pd-doped electrodes fabricated using sol-gel method exhibit less discharge capacity than samples of the non-doped LiFePO4/C electrode. The reduction in the level of performance is attributed to several reasons such as the shrinking of the lattice parameters, the formation of Li3PO4 impurity phase, and large particles size.;In contrast, for LiFePO4/C doped with Pt using sol-gel method, it was found that the crystallinity and the lattice parameters of LiFe 0.96Pt0.04PO4/C nanocomposite material increased when compared with LiFePO4/C electrode. No impurity was formed in the Pt-doped electrode. In addition, the Pt doped samples exhibited smaller particle sizes (100-200 nm) than the non doped electrode (200-500 nm). More homogeneous and uniform particles were also obtained with LiFe0.96Pt 0.04PO4/C than LiFePO4/C samples. Therefore, platinum doping might provide more space for the diffusion of Li+ ions which facilitates the movement of Li+ ions through the structure of LiFePO4/C material during the redox reactions in the battery, enhancing the discharge capacities.;In the third part of this study, LiFe1-xPdxPO 4/C (x = 0.00, 0.02, 0.04) and LiFe0.96Pt0.04PO 4/C nano composite cathode materials were synthesized by a hydrothermal method and the effects of Pd and Pt were examined. The results indicated that the optimized amount of palladium content (0.02%) in the electrode (LiFe 0.98Pd0.02PO4/C) reduced the nano composite particle sizes. This facilitates the Li+ ion diffusion and consequently enhances the reversibility and decreases the charge transfer resistance. The optimized Pd content in the electrode might act as a pillar to prevent the shrinking and collapse of the initial lattice structure. This might support the stabilization of the crystal structure during the intercalation/de-intercalation process of Li+ ions. However, by increasing the palladium content to 4%, the specific capacities decreased due to the Li3PO 4 impurity formation. The presence of such impurity may produce a small surface area for the redox reaction, and causes difficulty of Li+ ion diffusion during the redox electrochemical process. As a result, the optimized palladium doping is helpful in improving the electrochemical performance of LiFePO4/C material. A LiFe0.96Pt0.04PO 4/C based cathode nano material prepared by a hydrothermal method exhibited better performance when compared to the non-doped LiFePO4/C sample. The improvement in the electrochemical performances can be attributed to the combination of the following aspects related to the presence of Pt in the electrode. The platinum element can act as a stabilizing point of the crystal structure during the charge/discharge process. It contributes to the improvement of the redox reaction rate with the increase of the specific surface area of the composite electrode. LiFe0.96Pt0.04PO4/C electrode exhibited homogeneous small particles which might facilitate the Li+ ions diffusion rate.;In the first section, we determine the optimized amount of carbon support and morphology of the particles using SEM which help to obtain LiFePO 4/C cathode material with an excellent electrochemical performance. It was found that when the amount of coated carbon exceeds the optimized value, the discharge capacity of the LiFePO4/C material decreased. This might indicate a low diffusion of the Li+ ions through the carbon layers during the charge/discharge process. On the other hand, for LiFePO4 coated with carbon quantity lower than the optimum value, LiFePO4/C cathode exhibited poor capacity performance due to its low electrical conductivity. Therefore, both the quality and quantity of carbon coating on the surface of LiFePO4 particles are important and only optimized carbon content can lead to a more uniform carbon distribution. Optimized surface area and conductivity, which give high electrochemical performance, can be only achieved if the appropriate carbon content and method of electrode preparation are obtained.;The results show that the chemical and structural properties and electrochemical performances of Pd and Pt doped LiFePO4/C based electrodes to obtain LiFe1-xPdxPO4/C and LiFe1-xPt xPO4/C as Li-ion cathodes are significantly informed by the method of preparation of the materials and the doping element.